Hey, welcome to hubpages.
Basic idea is just write good, useful content and drive visitors to your hubs.
For the first part of it (content), you can pretty much write about whatever you want. Doesnt really matter what it is as long as you feel it's worth writing about. My suggestion though is that you write about things that people are interested in. Things that will be interesting for years and years to come so that you can continually benefit from your articles.
WIth regards to driving traffic. That's up to you how you do it but traffic is essential for you to make money here. There are 4 main ways you earn money:
Hubpages ad program
For the first two, you get capsules (little ad blocks which you can configure to promote a product) that you can use to sell products from these site. You earn money if you successfully promote them. So it's basically affiliate marketing if you're familiar with the term. Just write hubs that are in someway related to the products you wanna sell and then slap the products on there and you're good to go.
For the last two, you need to drive lots of traffic to your hubs. You earn from adsense if a reader clicks on one of the adsense ads. So it's a pay per click thing. The hubpages ad program pays per impression.
I earn the most, not much mind you (I still have too few hubs), from amazon and the hubpages ad program.
I suggest you head over to the learning center like what ThePracticalMommy said. Best you read up on the fine details of these things.
Good Luck and happy writing.